1. Field of the Invention
The invention relates generally to magnetic suspensions or levitations for moveable members. More specifically, the invention pertains to suspensions with unconstrained mobility in linear translation and a novel circuit using flux feedback for linearizing the net output armature force of a magnetic actuator adaptable to pointing and isolating instruments.
2. Description of the Prior Art
A differential magnetic actuator provides a single-axis, non-contacting force capability which is insensitive to cross-axis translations (i.e., those axes perpendicular to the direction of the force field). It includes at least two diametrically opposing electromagnets or half-stations acting on a moveable armature disposed therebetween. Force is exerted on the armature when a closed magnetic field is established around each half-station core and the moveable armature due to input coil current excitation. Force exerted by each half-station is unidirectional and acts only in a direction to decrease the gap between itself and the armature. By using two half-stations in a diametrically opposing configuration, the net armature force is equal to the algebraic sum of the force components contributed by each half-station.
The magnetic actuator exerts force on the armature along the axis perpendicular to the pole face. Armature motion along this axis is limited by the gap distance between each half-station. Translation along this axis would eventually result in contact between the armature and the actuator.
However, in the cross-axes (i.e. those axes perpendicular to the axis of force) armature or translation is not physically limited by the magnetic actuator. The only requirement is that the armature maintain a magnetic path between the half-station pole faces.
It is known to those skilled in the art that the net armature force of a differential magnetic actuator is inherently non-linear, being related to the squared functions of magnetizing currents and gaps for the co-energy relationship and to a squared function of gap flux densities for the energy relationship. Because of these inherent squared non-linearities, a significant problem arises in linearizing the net output armature force of the differential magnetic actuator in response to an applied force command; i.e., to obtain an armature force proportional to the input signal.
In providing linear force control on the armature of a differential magnetic actuator, previous control techniques have included Force Feedback and Current/Gap Feedback. See, for example, copending U.S. application Ser. No. 747,627, Position Sensor for Magnetic Suspension and Pointing System, invented by Brian J. Hamilton and assigned to the assignee of the present invention.
Force Feedback incorporates a force sensor in a closed-loop configuration to linearize the net forces on the armature. The armature is physically tied to the payload through the force sensor. Any force exerted on the armature is transmitted through the force sensor to the payload. The sensor itself is typically a quartz crystal which varies an oscillator frequency in response to a tensile or compressive force. Force on the armature is controlled by modulating the magnetic flux density produced by each half-station.
In this approach the force sensor, located between the suspended payload and armature, transmits a force signal across the magnetic gap, which through appropriate compensation, drives the flux producing coil current in each half-station. One drawback to this approach is that a harness assembly or complex non-contacting telemetry system must be employed across the magnetic gap. The use of a harness assembly is undesireable since it can degrade actuator performance by acting as a shunt spring.
Other drawbacks to this approach are that force sensors capable of a high bandwidth and resolution required for low level force control are costly, fragile, and require sophisticated support electronics. Further, because of their fragility, these sensors often require elaborate holding fixtures to protect them from damage during non-operational conditions.
As another approach, Current/Gap Feedback force linearization has been used. This technique is more common and utilizes the relationship between magnetizing current and air gap for a linear medium in an open force loop configuration. In this method force is controlled through the coenergy relationship that armature force is proportional to the square of the magnetizing current and inversely proportional to the square of the gap distance.
Any sensor capable of providing a signal proportional to gap position can be used. Previous applications have incorporated eddy current, capacitive, and inductive sensors.
By employing both current and gap position sensors, the requirement for and disadvantages of a force sensor are eliminated. To remove the current squared non-linearity, a bias current technique has been utilized. This consists of adding a flux producing bias current to one half-station and subtracting the same flux producing bias current from the diametrically opposite half-station.
However, because of the open force loop configuration and square law relationships, both the position and current sensing signals, as well as squaring compensation circuitry, must be very accurate and linear over full operational conditions. Because of the squaring relationship, percentage force errors can be greater than twice the percentage position and gap errors that cause them.
To implement the necessary gap feedback, position sensors are employed as described above. These sensors, because of their high accuracy requirements, are expensive and often require sophisticated compensation circuitry to achieve the necessary linearity over full operating conditions.
Additionally, because the applied armature force is related to the magnetic field established in the gap due the input coil currents, a linear relationship between the input coil current and magnetic flux is required. Therefore non-linearities due to hysteresis and saturation between the magnetic flux and input coil current can seriously impact actuator performance.
To attain the required linearity between the magnetic field and input coil current, core materials with low hysteresis properties are used. These materials however often require specialized processing and are difficult to machine, which raises the actuator cost. Also, these low hysteresis materials tend to saturate at relatively low flux densities. As a result a larger volume of core material is required to avoid saturation nonlinearities, increasing the actuator weight.
Finally, the Current/Gap Feedback force linearization requires multiplication circuitry which is often undesireable because of errors introduced due to temperature variations.
The present invention avoids the limitations of the prior art by utilizing the energy relationship between force and magnetic flux density in the gaps between the half-stations and armature to linearize the net force. Flux density is measured by a Hall Effect device.